Why is my Navel Piercing bleeding?

I've had my belly button pierced for four months now and I didn't see any problems in the first few weeks. The guy who did my piercing told me I could change my jewelry after two weeks, so I waited two weeks and changed it. There were no problems. Then out of the blue, the skin around the piercing turned red and sore. Now there's always nast looking crusty yellow buildup around the top ball and whenever I move the jewelry up or down, puss gushes out. I took the piercing out today to change it, (I cleaned everything completely before taking it out, while it was out and before I put the new one in. Between changing the jewelry, I squeezed the piercing in hopes to clean out all the nasty puss, but a painful jello-like bloody substance tried to come out. I stopped there and just put the new piercing in. I didn't have any problems with getting the jewelry in and it was painless so I'm not entirely sure what's going on.